1. Field of the Invention
This invention relates to the field of devices for suppressing transient electric disturbances on conductors, particularly low-voltage ac supply mains. The invention provides an electric surge suppressor especially for connection to branch circuits inside buildings, and includes a varistor in shunt with a resistor-capacitor damping network. This suppressor circuit is well coordinated with a suitable arrester and the suppressor circuit also provides damping of oscillatory surges.
2. Prior Art
Transient overvoltages, also called surges, that propagate on electric power conductors inside buildings are a common cause of damage and malfunction to electronic equipment, including computers, television receivers, video tape recorders, etc. Surge voltages typically measured in buildings may have durations ranging from sub-microseconds to a few milliseconds. Surge voltages can be unipolar or oscillatory, as described, for example in American National Standard C62.41-1991.
Unipolar overvoltages that impinge on a network of branch circuits often are converted to an oscillatory overvoltage by superposition of multiple reflections from impedance discontinuities along the branch circuit. Such discontinuities can be caused by electric loads that are connected along the branch circuit, as well by ends and junctions defined by the branch circuits. Unipolar overvoltages that impinge on branch circuits can also produce oscillations from the well-known response of an LC circuit produced by a combination of reactive loads, i.e., inductive and capacitative loads.
There have been numerous efforts to limit the peak magnitude of the surge voltage, and therefore to prevent damage and/or malfunction in electronic equipment. Most devices developed since about 1970 include one or more metal oxide varistor(s), silicon semiconductor(s), or filter circuit(s). Many of these devices are complicated and expensive to produce.
It is known to those skilled in the art that protection from surge voltages that is both effective and economical requires a combination of two different types of surge-protective devices. These two types are commonly known as arresters and suppressors.
An arrester is a heavy-duty surge-protective device that should be connected near the point at which the power conductors enter the building. Common places for the installation of an arrester include at the electric service meter, which measures kW.h consumption, or at the circuit breaker panel, both usually located near the entry point of the power conductors into the building. The purpose of an arrester is to divert large surge currents occurring with lightning strokes (e.g., more than 10 kA for between a few tens of microseconds and a few milliseconds). These surges can transfer more than 100 joules of energy to a surge-protective device. Surges characterized by such large energy deposition capability cannot be absorbed by low-cost surge-protective components.
A suppressor is a light-duty surge-protective device that should be installed between the branch circuit and the vulnerable equipment (e.g., a computer). The purpose of the suppressor is to further limit overvoltages at vulnerable equipment, especially small surges that either originate from switching loads inside the building or propagate down the power conductors from the arrester. Some models of suppressors also reduce the rate of change of voltage (dV/dt) at the equipment, which may help avoid temporary malfunction ("upset") of susceptible equipment that occurs without permanent damage.
In conventional applications, it has been common practice to specify the arrester with a greater conduction voltage than the suppressor. For example, one might use an arrester that limits the surge voltage to less than 2 kV and a suppressor that limits the surge voltage to less than 0.5 kV. The difference in voltage across the two surge-protective devices is the voltage drop along the resistance and inductance of the wire that connects them together, which typically might be a few tens of meters of wire. However, the resistance and inductance of the connecting wire, as well as the voltage protection levels of the conventional arrester and suppressor may not be coordinated. As a result, the voltage drop along the wire that connects the conventional arrester and suppressor can be inadequate. Thus the conventional suppressor is likely to conduct more current than the conventional arrester.
For surges with a current rate of change (dI/dt) of more than 0.5 kA/.mu.s, the inductance of the wire between the arrester and suppressor typically provides adequate coordination of the currents in these two protective devices. However, for surges with smaller values of dI/dt, such as commonly occur with surges with durations of more than 100 .mu.s, the impedance of the wire between the conventional arrester and suppressor may not be adequate to prevent excessive currents from flowing in the suppressor. It is possible for long-duration surges to cause surge suppressors to degrade or even to explode.
Recently, it has become clear that the best coordination is obtained by specifying that the surge arrester has a lower conduction voltage than the surge suppressor. In this way, the peak surge current in branch circuits inside a building can be limited to a relatively small value (e.g., less than 500 A), regardless of the duration of the surge or the impedance of the wire between the arrester and suppressor. By limiting the surge currents inside the building, one can reduce the transient electromagnetic fields radiated by the surge current and achieve improved electromagnetic compatibility. Large surge currents inside buildings are undesirable, because such large currents create large magnetic fields that may transfer energy electromagnetically to other conducting loops and thereby cause either damage or upset to electronic devices, including those not directly connected to the power conductors carrying the surge.
The present inventor has noted that conventional surge suppressors with low conduction voltages generally function to damp oscillatory responses in the branch circuit and at loads in the vicinity of the suppressor. As the conduction voltage of the suppressor is increased, in order to obtain better coordination, the problem of oscillatory overvoltages becomes worse.
In conventional practice there is an inability to achieve both excellent coordination with long-duration surges and suppression of oscillatory surges on branch circuits.